Some wall clouds have a feature similar to an "eye". Attached to many wall clouds, especially in moist environments, is a tail cloud, a ragged band of cloud and cloud tags (fractus) extending from the wall cloud toward the precipitation. It can be thought of as an extension of the wall cloud in that not only is it connected to the wall cloud but also that condensation forms for a similar reason. Cloud elements may be seen to be moving into the wall cloud, as it is an inflow feature. Most movement is horizontal, however, some rising motion is often apparent as well. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base, this feature is a collar cloud.
The wall cloud feature was first identified by Ted Fujita associated with tornadoes in tornadic storms. In the special case of a supercell thunderstorm but also occasionally with intense multicellular thunderstorms, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm which is most likely to produce tornadoes, and the vast majority of intense tornadoes.
Tornadogenesis is most likely when the wall cloud is persistent with rapid ascension and rotation. The wall cloud typically precedes tornadogenesis by ten to twenty minutes but may be as little as one minute or more than an hour. Often, the degree of ascension and rotation increase markedly shortly before tornadogenesis, and sometimes the wall cloud will descend and "bulk" or "tighten". Tornadic wall clouds tend to have strong, persistent, and warm inflow air. This should be sensible at the surface if one is in the inflow region, in the Northern Hemisphere, this is typically to the south and southeast of the wall cloud. Large tornadoes tend to come from larger wall clouds.
Although it is rotating wall clouds that contain most strong tornadoes, many rotating wall clouds do not produce tornadoes. Absent a low-level boundary, tornadoes very rarely occur without a rear flank downdraft (RFD), which usually manifests itself visually as a drying out of clouds, called a clear slot or notch. The RFD initiates the tornado, occludes around the mesocyclone, and when it wraps completely around, cuts off the inflow causing death of the low-level mesocyclone and tornadolysis. Therefore, in most cases, the RFD is responsible for both the birth and the death of a tornado.
Usually, but not always, the dry slot occlusion is visible (assuming one's line of sight isn't blocked by precipitation) throughout the tornado life cycle. The wall cloud withers and will often be gone by the time the tornado lifts. If conditions are favorable, often even before the original tornado lifts, another wall cloud and occasionally a tornado may form downwind of the old wall cloud, typically to the east or the southeast in the Northern Hemisphere.
The LaGrange Tornado during VORTEX2. Part II: Photogrammetric Analysis of the Tornado Combined with Dual-Doppler Radar Data
Sep 01, 2012; ABSTRACT This study presents the synthesis of dual-Doppler and cloud photography data of the 5 June 2009 Goshen County, Wyoming,...